The present invention relates to a method for forming an extrudate such as by applying a coating on a substrate. In particular, the invention applies to a method of preparing a die to improve the uniformity of the extrudate.
The production of high quality articles, particularly electronic, tape, optical, photographic, photothermographic, thermographic, abrasives, adhesive, display, and pharmaceutical articles, consists of applying a thin film of a coating solution onto a continuously moving substrate or web. Thin films can be applied using a variety of techniques, including dip coating, forward and reverse roll coating, wire wound rod coating, and die type coating. Die coaters include knife coaters, slot coaters, slide coaters, slide curtain coaters, drop die curtain coaters, and extrusion coaters among others. Many types of die coaters are described in the literature such as by Edward Cohen and Edgar Gutoff, Modern Coating and Drying Technology, VCH Publishers, NY 1992, ISBN 3-527-28246-7 and Gutoff and Cohen, Coating and Drying Defects: Troubleshooting Operating Problems, Wiley Interscience, NY, ISBN 0-471-59810-0.
Die coating is a process whereby a pressurized stream of coating material is moved through an internal manifold of a die coater and discharged from an exit slot to form a ribbon of coating material. The uniformity of the coating layer depends on the precision of the coating slot of the coating dies through which the coating layer (or extrudate) passes. Coatings can be applied as a single layer or as two or more superimposed layers. Although it is usually most convenient for the substrate to be in the form of a continuous web, it may also be formed to a succession of discrete sheets. In the manufacturing of coated products, it is generally desirable to attain a uniform coating thickness having the smallest possible variation in the coating thickness. This is especially true for products for critical optical or electronic applications.
All die coaters have at least one slot. Each slot has a slot width corresponding to the coated width, a slot length corresponding to the distance from the manifold cavity to the exit of the slot, and a slot height, which is the narrow dimension of the slot between the two parallel surfaces defining the slot itself. One of the fundamental problems associated with die type coaters is the ability to accomplish a uniform flow per unit width across the entire width of the slot of the die. A fundamental issue in attaining this uniformity of the flow, and thus the critical uniformity of the coated product, is the ability to construct a die with the best possible precision of the die slot xe2x80x9cheightxe2x80x9d (e.g., a uniform die slot xe2x80x9cheightxe2x80x9d across the entire width of the slot).
In an attempt to improve coating uniformity, various manual, mechanical, thermomechanical, piezomechanical, magnetostrictive, and motor driven actuators have been installed on coating dies to control the die slot. The actuators can be located to generate an individual displacement force locally across the width of the slot exit. Since at any point across the die width the local discharge rate from the slot exit depends on the local gap, the uniformity of the flow rate from the die can be controlled across the width. U.S. Pat. No. 5,587,184 discloses a coating die with a slot thickness control mechanism located away from the slot exit.
Control of the die slot is typically accomplished by measuring the thickness of the film or coating at various points across its width with a thickness gauge such as a beta-ray, x-ray, or light absorption gauge. With the information from such measurements, an operator can manually adjust a bolt-type actuator bearing against the coating die. Alternatively, a control system can signal the activation of actuators which bear against the coating die or which rotate bolts that bear against the coating die. The manual adjustment of the coating die flexing bolts by an operator requires skill and experience. It has been shown that the quality of the product extruded or coated can be improved by a closed loop control system to replace the manual operator adjustment.
The die slot is typically not set for optimum uniformity when initially assembled. The adjustment cycle is time consuming and typically results in significant waste of coating material and substrate. Moreover, the actuators are not truly independent, but interact. That is, an adjustment of one actuator can require an adjustment of adjacent actuators. Consequently, the cross machine direction (crossweb) mechanical resolution, coupled with the limitations discussed above, results in inadequate accuracy of the die slot.
The invention is a method of forming extrudate having substantially uniform thickness. A shim is formed which has a thickness variation of less than or equal to about 0.5 mil (13 microns) total indicated runout. This shim is disposed between a first die portion having a first surface and a second die portion having a second surface. A slot is formed between the first surface and the second surface. The slot has a height dimension substantially the same as the shim thickness. Flowable material is extruded through the slot.